A tuning fork gyro, such as is shown in FIG. 1, induces oscillation of a proof mass 10 in a drive direction 12. Rotational movement in direction 14 perpendicular to the drive direction 12 results in acceleration of the proof mass 10 in a sense direction 16 perpendicular to the drive direction 12 in a phenomenon known as Coriolis' force. In the typical set up, two proof masses 10 are used. The two proof masses 10 oscillate 180 degrees out of phase, such that they are always moving in opposite directions. As a result, the Coriolis's force imposed on the proof masses 10 will also be in opposite directions.
Movement of the proof mass 10 along the sense direction 16 changes the capacitance between the proof mass 10 and a sense plate 18 positioned below the proof mass 10. The sense plate 18 is maintained at a constant voltage (±VS) such that changes in capacitance will induce a voltage in the proof mass 10. The proof mass 10 is connected to a charge amp 20, which converts the current induced by the changing capacitance into an output voltage (VO). VO may therefore be used to calculate the extent of Coriolis' force experienced by the proof mass 10.
Oscillation is induced by a drive comb 22 and pick-off comb 24. An alternating current (AC) drive voltage (VD+,VD−) applied to the drive comb 22 attracts the proof mass 10 due to capacitative charge build-up on the drive comb 22 and proof mass 10. Movement of the proof-mass 10 changes the capacitance between the proof mass 10 and the pick-off comb 24, resulting in a signal (VPO), which is fed back to a drive circuit 26 and used to determine the magnitude and phase of VD effective to cause the proof mass 10 to oscillate.
The proof mass 10 is elastically mounted to a substrate 28 by means of flexures 30 secured to the substrate 28 by means of pads 31. Due to imperfections in the flexures 30, oscillation of the proof mass 10 induced by VD is not entirely parallel to the drive direction 12. Instead, the proof mass 10 moves along a trajectory having components parallel to both the drive direction 12 and the sense direction 16. The extent of this movement along the sense direction 16 is often large enough to saturate the output of the gyro such that movement along the sense direction 16 induced by Coriolis' force cannot be distinguished.
Prior systems compensated for out-of-plane movement caused by the flexures by adding a direct current (DC) bias voltage (VB) to VD. As the proof mass 10 oscillates relative to the drive comb 24, the voltage VB applied to the drive comb 24 induces an oscillating current flow in the proof mass 10 due to changes in the capacitative distance between the drive comb 24 and proof mass 10. The value of VB is selected such that the magnitude of the current flow induced in the proof mass 10 has the opposite polarity but the same magnitude as current flow induced by out-of-plane movement caused by imperfections in the flexures 30.
The primary weakness of this system is that imperfections in the input signal (VD+VB) pass directly to the proof mass 10 and therefore to VO. As a result, bias, noise, and the like, caused by imperfections in the drive electronics show up in the output signal. Inasmuch as VD and VB are both large relative to the signals generated by Coriolis' force, the noise inherent in them significantly degrades the accuracy of measurements.
It would therefore be an advancement in the art to provide a tuning fork gyro compensating for out-of-plane movement induced by flexure imperfections without introducing input signal noise into the output signal.